1. Field of the Invention
The present invention relates to a noise-reduction system for reducing the noise generated from the tank of a stationary induction apparatus such as a power transformer or a shunt reactor. It is a particular implementation of an "Active Acoustic Transmission Loss Box" described in U.S. patent application Ser. No. PCT/US92/08401 filed 8 Oct. 1992.
2. Background Art
Stationary induction apparatus such as power transformers and shunt reactors are used in utility substations and elsewhere for electric power transmission. These devices produce a low-frequency hum that is a source of noise pollution for persons working or living near the substations. The noise is due to magnetostriction of the core being transmitted to the tank (either directly or through the oil). The vibrating tank in turn radiates acoustic energy to the far field. The stationary induction apparatus in North America generate 120 Hz tones (plus harmonics of the 120 Hz fundamental).
Passive techniques have been tried to decrease noise of stationary induction apparatus with only limited success. One method requires surrounding the transformer or reactor with an expensive masonry building. Another approach discussed by Minoru Kanoi, et al., "Noise Reduction Device for Stationary Induction Apparatus," U.S. Pat. No. 4,514,714 dated Apr. 30, 1985, incorporated by reference herein, requires completely covering the tank with complex, multilayer sound-damping panels. Each of these panels also require finely tuned absorbers. Both of these approaches are expensive and limit maintenance and repair. In addition, the passive panels are not suitable for use as a retrofit. Noise reduction is limited to only about 10 dBA with these passive techniques.
Active techniques also have been tried to decrease noise of stationary induction apparatus with only limited success. For example, Conover, "Noise Reducing System for Transformers," U.S. Pat. No. 2,776,020, Jan. 1, 1957, incorporated by reference herein, mounted an array of loudspeakers along one side of a transformer and was able to reduce the noise from one side of the transformer for brief periods of time, at which time his circuitry required readjustment. More recently Angevine "Active Cancellation of the Hum of Large Electric Transformers," Proceedings of Inter-noise '92, July 20-22, 1992, Toronto, Canada, incorporated by reference herein, repeated this experiment using an array of 8 loudspeakers, and two 4-channel, adaptive controllers to adjust the signals to the loudspeakers. Angevine used loudspeakers located a few meters from the transformer, and microphones about 30 m from the transformer. It is very difficult to obtain adequate noise measurements with the microphones so far removed from the transformer. Low background noise is required to obtain adequate signal-to-noise ratios. Small amounts of wind or small changes in temperature will degrade the transfer function measurement. Angevine reports limited reduction over a narrow angle (30.degree.) or less. Angevine reports degraded performance with wind or thermal changes. This approach is of little commercial utility because it does not provide continuous, global cancellation of the transformer noise, and is physically obstructive.
Another active method that has been tried with limited success is the use of vibrators attached to the tank walls as discussed in U.S. Pat. No. 4,435,751. The difficulty with this approach is that the tank vibration can be decreased locally where the vibrator is attached, but the vibration invariably increases elsewhere on the tank when canceling the first harmonic. This difficulty is due to a volumetric change in the transformer core at the fundamental frequency of the magnetostriction. Since the transformer oil is essentially incompressible, decreasing the vibration at one point on the tank likely results in increasing the vibration in other uncontrolled areas of the tank. Controlling the entire tank surface is not practical, with the net result that global noise reduction is not obtained.
What is required to obtain continuous, global cancellation of the transformer noise is a particular implementation of an "Active Acoustic Transmission Loss Box" as described by Fuller, McLoughlin and Hildebrand in PCT Application PCT/US92/08401 filed 8 Oct. 1992 (incorporated by reference herein), together with a multiple-interactive, self-adapting controller, as described by Tretter, "Repetitive Phenomena Cancellation Arrangement with Multiple Sensors and Actuators," U.S. Pat. No. 5,091,953 dated Feb. 25, 1992 (incorporated by reference herein). As described by Fuller et al, active enclosures and active panels can be used to reduce noise from machinery such as power transformers. However, there are certain unique and unusual modifications that are required for the "Active Acoustic Transmission Loss Box" to be commercially successful for the control of noise from power transformers and similar machinery.